Method for transmitting a binary data signal to or from a satellite via an optical feeder link

ABSTRACT

A method for transmitting a binary data signal to or from a satellite via an optical feeder link, wherein an optical transmitting interface carries out the following steps multiplexing binary physical layer frames which are associated with a plurality of carriers or a plurality of beams in a satellite communication system into a single bit stream, inserting a binary physical layer frame identification sequence upstream of each physical layer frame, wherein the physical layer frame identification sequence comprises: a unique binary synchronization sequence, a binary beam index sequence, a binary carrier frequency index sequence, a binary baud rate index sequence, a binary roll-off factor index sequence, a binary modulation index sequence.

FIELD OF THE INVENTION

The present invention relates to a method for transmitting a binary datasignal to or from a satellite via an optical feeder link.

BACKGROUND OF THE INVENTION Description of Related Art

In the forward link of a satellite a gateway transfers data signals tothe users using a satellite. Such a scenario is illustrated in FIG. 1.

Such systems as well as corresponding transmitting methods areillustrated in the following publications:

-   [1] “ICT-2011.1.1 BATS D4.1: Satellite Network Mission    Requirements”, Broadband Access via Integrated Terrestrial and    Satellite Systems (BATS) European Project, Tech. Rep., 2012.-   [2] “Optimised Smart Gateway Diversity for BATS”, Broadband Access    via Integrated Terrestrial and Satellite Systems (BATS) European    Project, BATS Factsheet 8, May 2014.-   [3] Second Generation Framing Structure, Channel Coding and    Modulation Systems for Broadcasting, Interactive Services, News    Gathering and Other Broadband Satellite Applications; Part II:    S2-Extensions (DVBS2X), Digital Video Broadcasting 30 (DVB) Std.    ETSI EN 302 307-2, October 2014.-   [4] Implementation Guidelines for the Second Generation System for    Broadcasting, Interactive Services, News Gathering and Other    Broadband Satellite Applications; Part II: S2-Extensions (DVB-S2X),    Digital Video Broadcasting (DVB) Std. ETSI TR 102 376-2, March 2015.

A satellite system known from prior art, such as that described inpublication 1, comprises a number of gateways N_(gw) which serve anumber of spot beams. In the case of a large number of spot beams (up to151 spot beams per satellite as per publication [2], for example) alarge number of gateways (up to 29 gateways, see publication [2]) isrequired in the radio frequency (RF) V-band, for example, due to thelimitation of the spectrum in the feeder link.

The setup of the transmitting system in the feeder link, as is knownfrom prior art, is illustrated in FIG. 2. In the forward link standardDVB-S2X as per publication 3 one or a plurality of carriers areassociated with a user beam. For example, in the broadband multicarrierreference scenario, which is specified in publication 4, three carriersof approximately 500 MHz each are associated with a user beam. Thisresults in a 1.5-GHz spectrum per user beam, as is illustrated in FIG.3. Each carrier is modulated by a pulse-shaped stream of super frameswhich consist of bunched physical layer frames. Taking intoconsideration the RF-V-band which is used in the feeder link, only twobeams can be accommodated in the spectrum, which results in a strongincrease in the number of required gateways.

Each carrier bit stream is framed in physical layer frames. After amodulation using amplitude and phase shift keying (APSK) and pulseshaping by a square root raised cosine filter (SRRCF) up to threeDVB-S2X carriers per beam are multiplexed. After an upward conversion tothe carrier frequency in the RF-V-band the signal is amplified by ahigh-performance amplifier and transmitted via the gateway antenna.

In the satellite the signal is received by the feeder link antenna. Thebeam currents are filtered, amplified and frequency-converted to thecarrier frequency in the user link in the RF Ka-band using N_(T)transponders. The signal is then transferred to the N_(u) user terminalsby the user link antenna.

The return link carries out the reverse operation and the return feederlink is associated with the RF Q-band.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method fortransmitting a data signal to or from a satellite, wherein a lowernumber of gateways is required. Further, a method for receiving andprocessing a binary data signal in the satellite communication is to beprovided.

In the method according to the present invention a binary data signal istransmitted to or from a satellite via an optical feeder link. For thispurpose the following steps are carried out in an optical transmittinginterface:

multiplexing binary physical layer frames which are associated with oneof a plurality of carriers and a plurality of beams in the satellitecommunication system into a single bit stream,

inserting a binary physical layer frame identification sequence upstreamof each physical layer frame, wherein the physical layer frameidentification sequence comprises:

a unique binary synchronization sequence,

a binary beam index sequence,

a binary carrier frequency index sequence,

a binary baud rate index sequence,

a binary roll-off factor index sequence,

a binary modulation index sequence.

The method according to the present invention is used in the feeder linkof a satellite, wherein the data communication can take place eithertowards the satellite or vice versa. This means that either thesatellite can be the transmitter and the gateway can be the receiver orvice versa. On both sides of the transmission path an optical interfaceis located, namely on the one hand an optical transmitting interfacewhich has already been described and on the other hand an opticalreceiver interface which will be described in detail below. Preferably,this interface is arranged downstream of the framing block where thebitwise representation of the physical layer frames is generated in eachcarrier per beam. In order to obtain the information concerning thestructure of the underlying carrier bit stream and further maintain aproper association of the physical layer frames to the carriers andbeams the described physical layer frame identification sequence isused.

The length of the stated bit sequences can be selected such that enoughbits are available for transmitting the required information.

In a preferred embodiment, at least some sequences of the physical layerframe identification sequence comprise redundancy bits in order to beless susceptible to sporadic interferences on the transmission channel.

In a further preferred embodiment, the optical transmitting interfacecomprises a data buffer for storing the binary bit stream of thephysical layer frame to compensate for differences in the baud rate ofthe carriers.

Preferably, the physical layer frames remain unchanged such that themethod remains open to future modifications of the DVB-S2X transmissionstandards.

It is further preferred that the binary data stream is used at theoutput of the optical transmitting interface for modulating an opticalcarrier, for example a laser. The optical carriers are then associatedwith a dense wavelength division multiplexing (DWDM) grid. Depending onthe magnitude of the selected DWDM grid a number of beams (N beams) canbe associated with the DWDM channel.

For example, the DWDM-multiplexed optical signal is amplified by anerbium-doped fiber amplifier (EDFA) and transmitted via the feeder linkusing the optical telescope transmitter.

Preferably, a coherent modulation, in particular an amplitude or phasemodulation, is used for modulating the optical carrier signal.

Alternatively, an incoherent intensity modulation, in particular a pulseamplitude or pulse position modulation, can be used for modulating theoptical carrier signal.

The present invention further relates to a method for receiving andprocessing a binary data signal which is transmitted to or from asatellite via an optical feeder link. The transmission can take placeaccording to the method described above. An optical receiving interfacecarries out the following steps:

synchronizing the binary data stream using the unique binarysynchronization sequence,

extracting the corresponding physical layer frames.

On the satellite side the optical signal is thus received by a receivingtelescope. The signal can again be amplified by an EDFA. Downstream of aDWDM demultiplexer each optical carrier in the DWDM channel can bedemodulated by a photodiode and a demodulator for restoring the digitalbit stream.

In addition, any redundancy bits in the physical layer frameidentification sequence can be removed.

Since the physical layer data stream comprises a structure in which eachphysical layer frame identifier is followed by a physical layer frame,the frames which are associated with a DVB-S2X carrier can be restoredfrom this data stream. The unique synchronization bit sequence allowsfor extraction of the physical layer frames. They are then modulatedaccording to the modulation format which is given in the physical layerframe identifier without the physical layer header having to be decoded.

Preferably, the physical layer frames are modulated and processed bypulse shaping to produce a carrier signal according to the modulationindex, the roll-off factor index and the baud rate index in the physicallayer frame identification sequence.

The DVB-S2X carriers can subsequently be multiplexed and upwardconverted according to the carrier frequency index and the beam index inthe physical layer frame identification sequence. For instance, theupward conversion can be performed to a frequency which is used in theuser link, for example the Ka-band.

The beam signals can then be filtered and amplified to be then sent tothe user via the user link.

On both sides of the transmission path the optical interfaces can beimplemented by FPGA modules.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, enabling one of ordinary skill in the art to carryout the invention, is set forth in greater detail in the followingdescription, including reference to the accompanying drawing in which:

FIG. 1 shows a schematic representation of a forward link in thesatellite communication,

FIG. 2 shows the transmission chain of a satellite forward linkaccording to prior art,

FIG. 3 shows the association in the carrier spectrum in the RF V-bandaccording to prior art,

FIG. 4 shows an embodiment of the transmission chain according to thepresent invention,

FIG. 5 shows the association of the beams in a DWDM channel in a methodaccording to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIGS. 1 to 3 have already been elucidated in the context of prior art.

In FIG. 4 an embodiment of the optical transmission chain of the methodaccording to the present invention is illustrated. The data coming froma network are supplied to a framing block via a network interface. Thebinary data stream at the output of the optical gateway interfaces isused for modulating an optical carrier signal of a laser in a DWDMchannel. After the multiplexing and amplification the signal is suppliedto the telescope and sent via the transmission channel.

On the receiving side a processing of the signal takes place in areverse order, as has already been illustrated in the presentapplication. This is shown in the upper part of FIG. 2.

The association of the beams in a DWDM channel in the form of amultiplexed physical layer bit stream is illustrated in FIG. 5. Therepresentation includes the physical layer frame identification sequenceI1 to IN which is arranged upstream of each physical layer frame. Thissequence includes the following information:

a unique binary synchronization sequence (S),

a binary beam index sequence (BI),

a binary carrier frequency index sequence (CF),

a binary baud rate index sequence (BR),

a binary roll-off factor index sequence (RO),

a binary modulation index sequence (M).

The length of the stated fields is system-specific, wherein thisinformation should be present at the receiver end. The synchronizationsequence (S) can comprise any length which is sufficient to allow for areliable identification of the start of the physical layer frameidentification sequence. For example, this can be realized by acorrelation with the known sequence at the receiver. An example of asynchronization sequence in which a 32-bit synchronization word is usedis illustrated hereunder:

-   -   11110011001110110111110010001111

The next field identifies which beam is associated with the physicallayer frame. In a system having 150 beams, as described in the caseshown in FIGS. 1 and 2, for example, 8 bits are sufficient forrepresenting this information. An example of the BI field for beam 67using a decimal-to-binary conversion is as follows:

-   -   01000011

Depending on the number of carriers which are associated with a beam thelength of the carrier frequency field is defined. In a system havingthree carriers per beam, as is described in publications 1 to 4, forexample, 2 bits are sufficient. An example of the CF field of the secondcarrier using a decimal-to-binary conversion is as follows:

-   -   10

Since the baud rate may be variable a 64-bit double-precisionfloating-point number is used in the BR field.

An example of a 425 Msps carrier is:

-   -   0100000110111001010101001111110001000000000000000000000000000000

In addition to the baud rate the roll-off factor of the carrier in theRO field is given. For a system having six different options forroll-off factors (as per publications [3] and [4] 5%, 10%, 15%, 20%, 25%and 35%) 3 bits are sufficient for giving the stated information in theRO field. When the roll-off factors are used in the stated order anexemplary roll-off factor of 20% using a decimal-to-binary conversionwould be:

-   -   100

Finally, the MODCOD index is given in the M field. Proceeding from the12 MOD-CODs which are specified in DVB-S2X (see publications [3] and[4]) 7 bits are sufficient for including this information. An example ofthe MODCOD with index 45 using a decimal-to-binary conversion is:

-   -   0101101

As a result the overall bit number in the physical layer frameidentifier is 116 bits. Proceeding from a length of a short physicallayer frame of 16,200 symbols an overhead of 0.72% is achieved which isthus negligible.

According to the definition of the physical layer frame identifier thecomposition of the bit stream to be transmitted is observed across theoptical feeder link. A satellite having a high throughput can serve upto 150 user beams (see publication [2]). The data traffic in each beamis associated with three carriers having a symbol rate of 425 Msps, forexample, which results in an overall symbol rate of 1275 Msps per beam.Using the MODCOD with the highest modulation order and the highestcoding overhead, for example 256 APSK having a code rate of 29/45, amaximum 15 bit rate of approximately 15.8 Gbps per beam can be achievedusing a 0.4 nm DWDM grid. With 50-GHz channels the data traffic forthree beams can be multiplexed. The multiplexed bit stream is modulatedon the optical carrier (the laser) using a coherent QPSK modulation. Theresultant optical signal is supplied to a DWDM channel.

The required overall bandwidth for all 150 beams thus amounts to(0.4×150/3) nm=20 nm. As a result the overall data traffic in theforward feeder link can be readily arranged within the optical L-bandbetween 1,565 nm and 1,625 nm such that merely a single optical gatewayis required. This is a significant reduction as compared with the 29 RFQ/V-band gateways which are required as per publication 2.

Although the invention has been described and illustrated with referenceto specific illustrative embodiments thereof, it is not intended thatthe invention be limited to those illustrative embodiments. Thoseskilled in the art will recognize that variations and modifications canbe made without departing from the true scope of the invention asdefined by the claims that follow. It is therefore intended to includewithin the invention all such variations and modifications as fallwithin the scope of the appended claims and equivalents thereof.

1. A method for receiving and processing a binary data signal which istransmitted to or from a satellite via an optical feeder link, whereinan optical receiving interface carries out the following stepscomprising: synchronizing the binary data stream using the unique binarysynchronization sequence, extracting the corresponding physical layerframes.
 2. The method according to claim 1, wherein an additional stepis provided: removing the redundancy bits in the physical layer frameidentification sequence.
 3. The method according to claim 1, whereineach optical carrier in the DWDM channel is demodulated by a photodiodeand a demodulator for restoring the digital bit stream.
 4. The methodaccording to claim 1, wherein the physical layer frames are modulatedand processed by pulse shaping for producing a carrier signal, accordingto the modulation index, the roll-off factor index and the baud rateindex in the physical layer frame identification sequence.
 5. The methodaccording to claim 1, wherein the carriers are multiplexed and upwardconverted according to the carrier frequency index and the beam index inthe physical layer frame identification sequence.